Issue link: https://iconnect007.uberflip.com/i/1478618
SEPTEMBER 2022 I DESIGN007 MAGAZINE 29 • e propagation speed of microstrip is always faster than stripline—typically by 13-17%. • e speed of propagation of digital signals is independent of trace geometry and impedance. • Drivers do not have the same impedance as the transmission line, so terminations are used to balance the impedance. DESIGN007 Resources 1. Beyond Design by Barry Olney: Signal Flight Time Variance in Multilayer PCBs; Stackup Planning Parts 1-6; The Fundamental Rules of High-Speed PCB Design Part 2; The Impact of PDN Impedance on EMI. Barry Olney is managing direc- tor of In-Circuit Design Pty Ltd (iCD), Australia, a PCB design service bureau that specializes in board-level simulation. The company developed the iCD Design Integrity software incorporating the iCD Stackup, PDN, and CPW Planner. The software can be downloaded at www.icd.com.au. To read past columns, click here. • For DDR type designs, start with closely coupled plane pairs on the second and third layers from the top and bottom. ese plane pairs should be positioned in the stackup close to and directly below/ above the IC. • e ability to extract the plane pair data and via size from the stackup to a PDN planner allows one to see the plane resonance and the impact it has on the AC impedance of the PDN. • e AC impedance of the PDN should be kept below the target impedance up to the fih harmonics of the fundamental. • Plane resonance can be dampened by adjustments to the dielectric constant and thickness as well as plane size to avoid anti-resonance peaks. • If critical signals are routed on the outer microstrip layers, then there will be a large discrepancy in timing due to the variance in flight time through different materials of the outer microstrip and inner stripline layers. Figure 6: Matching the Spartan 6 driver to the transmission line. (Source: iCD Termination Planner)